how can I update column value before SaveChanges()

Hi, 

how can I update column value before SaveChanges(); when ActionResult httpost run

for this column:

Database.Tablolar.Kullanicilar.CreatedDate = DateTime.Now

(Sorry for my easy question, I am new here :) for mvc)

[HttpPost]
        public ActionResult Register(Kullanicilar kullanicilar)
        {
            if (ModelState.IsValid)
            {

                _context.Kullanicilar.Add(kullanicilar);
                _context.SaveChanges();

                ModelState.Clear();
                ViewBag.Message = kullanicilar.Adi + " " + kullanicilar.Soyadi + " kaydınız tamamlandı.";

            }
            return View();
        }

Table

CREATE TABLE [dbo].[Kullanicilar] (
    [Id]             INT            IDENTITY (1, 1) NOT NULL,
    [Adi]            NVARCHAR (50)  NOT NULL,
    [CariKodu]       NVARCHAR (40)  NULL,
    [Cep]            NVARCHAR (15)  NOT NULL,
    [CreatedDate]    DATETIME2 (7)  NOT NULL,
    [CreatedUserId]  INT            NOT NULL,
    [Eposta]         NVARCHAR (MAX) NOT NULL,
    [Ip]             NVARCHAR (15)  NULL,
    [PicturedId]     INT            NOT NULL,
    [Session]        NVARCHAR (MAX) NULL,
    [SifreDogrulama] NVARCHAR (MAX) NOT NULL,
    [Sifresi]        NVARCHAR (MAX) NOT NULL,
    [Soyadi]         NVARCHAR (50)  NOT NULL,
    [UpdatedDate]    DATETIME2 (7)  NOT NULL,
    [UpdatedUserId]  INT            NOT NULL
);